Conventionally, a sputtering method has been used as one of the methods for manufacturing thin films. The basic structure of the sputtering apparatus used for such sputtering method is described in "Basic Technology of Thin Film" (written by Akira Kinbara, published by Hideo Tanaka, printed by the Tokyo University Publishing Foundation, 1985. 5. 20, Vol. 8, pages 53 to 56).
As shown in FIG. 5, in this apparatus, a cathode 2 and an anode 3 are provided in a vacuum film forming chamber 1; a target member 4 is attached to the cathode 2 and a substrate 5 is attached to the anode 3; and the predetermined amount of gas is introduced into the vacuum chamber 1 via a gas bomb (a gas cylinder) 6, a mass flow controller 7 and a pressure controlling valve 8. In this state, glow discharge is conducted by applying voltage between the cathode 2 and the anode 3; and a material for a film, which is sputtered from the target member 4 by using plasma generated by the glow discharge as an energy source, are deposited onto the substrate 5. These sputtering apparatus are divided into two types: a direct current (DC) type sputtering apparatus in which direct current is used for the power source, and a radio frequency (RF) type sputtering apparatus in which radio frequency is used for the power source. Moreover, in order to obtain high density plasma, a magnetic field convergence (magnetron discharge) type apparatus is used. In this magnetic field convergence type apparatus, electron released from a cathode does not go directly to an anode but is trapped in a space having both electric field and magnetic field which are crossing at right angle each other. By such structure, high density plasma can be provided and the efficiency of forming films can be enhanced.
In recent years, magnetron discharge type sputtering apparatus have been combined with direct current type or radio frequency type apparatus. The direct current type apparatus permits sputtering dielectric materials, and the radio frequency type apparatus permits sputtering both insulating materials and conductive materials. Which apparatus to be used is determined depending upon target members to be sputtered.
The above mentioned sputtering method is used for forming each layer of an optical disc. In general, a direct current (DC) magnetron discharge type apparatus is used for forming a recording thin film and a reflecting layer of an optical disc. Radio frequency (RF) magnetron discharge type apparatus may be used for forming the protective layer comprising insulating materials. At present, rewritable optical discs such as a magneto-optical disc, a phase change optical disc or the like, have been developed and put into practical use. The basic structure of these optical discs includes a first protective layer, a recording thin film, a second protective layer and a reflecting layer which are sequentially formed on a transparent substrate which comprises guide grooves for laser beams. In general, an overcoat layer comprising UV-ray curing resins is provided on the reflecting layer to prevent films from being damaged. In the above mentioned phase change optical disc, recording is conducted by making the recording film amorphous by heating a recording film to a temperature which is not less than the melting point and then quenching the recording film. Erasing is conducted by crystallizing the recording film by heating the recording film to a temperature which is not less than crystallization temperature nor more than melting point and then cooling slowly thereof. Signals are obtained by the difference in reflectance between the amorphous state and the crystalline state.
With phase change optical disc, where a Ge--Sb--Te type chalcogen compound is used for a recording film, the melting point is not less than 600.degree. C. Thus, the protective film provided on both sides of the recording film is subjected to a thermal stress every time recording and erasing are conducted. Therefore, to provide a protective layer, a thermally stable film which has excellent adhesion to the recording thin film and having a strong mechanical strength is required. One protective layer which satisfies this conditions is a protective layer comprising a mixture of ZnS and SiO.sub.2. However, even if films are formed by the use of a mixture of ZnS and SiO.sub.2, the properties of the protective layer may change due to process conditions. For example, the environment for storing the substrate, exhausting time or the like may affect the properties of the overwrite cycle of recording and erasing an optical disc, which is manufactured through the step of forming film protective layer. In particular, the difference is made in the number of overwritable cycles between discs manufactured in high humidity and discs manufactured in low humidity. The discs manufactured in low humidity do not have the stable overwrite cycle property.
The object of the present invention is to solve the above mentioned problems and to provide a method for manufacturing an optical recording medium which is usually stable and is not affected by environmental conditions such as humidity condition and to provide a film forming apparatus used therefor.